Varnish for laminate or prepreg, laminate or prepreg prepared using this varnish, and printed wiring board prepared using this laminate or prepreg

ABSTRACT

Disclosed are a varnish for laminate or prepreg, comprising a heat treatment product which is obtained by mixing together (a) an epoxy resin, (b) dicyandiamide, and (c) a compound having an imidazole ring so that the component (c) is present in an amount of 0.001 to 0.03% by weight, based on the weight of the component (a), and subjecting the resultant mixture to reaction for heat treatment in an organic solvent at a temperature of 70° C. to less than 140° C. so that all of the components are compatible with one another in the absence of a solvent; and (d) inorganic filler, a laminate or prepreg prepared using the varnish and a printed wiring board prepared using the laminate and/or prepreg.

This application is a Continuation application of application Ser. No.10/450,176, filed Jun. 12, 2003, the contents of which are incorporatedherein by reference in their entirety. No. 10/450,176 is a NationalStage application, filed under 35 USC 371, of International (PCT)Application No. PCT/JP01/10884, filed Dec. 12, 2001.

TECHNICAL FIELD

The present invention relates to a varnish for flame-retardant laminateor prepreg for electrical insulation, a flame-retardant laminate orprepreg for electrical insulation obtained by using the varnish, and aprinted wiring board using these materials.

BACKGROUND ART

In recent years, there are demands for laminates (which, in the presentinvention, include a laminate covered with a metal foil on the top orthe top and bottom, namely, a metal-clad laminate) and prepregs forelectrical insulation, in which they should have a high stiffness and ahigh dielectric constant or contrary a low dielectric constant, and befree of halogens, and the laminates and prepregs tend to contain variousinorganic fillers in a large amount for meeting such demands. Especiallythe production of halogen-free laminates or prepregs is carried out foravoiding a so-called dioxin problem that arises when burning materialscontaining halogenated compounds, and a large amount of an inorganicoxide or hydroxide is frequently added to impart flame retardancy to alaminate or prepreg without using a halogenated flame retardant.

However, when a large amount of inorganic filler is added to the resinsystem which has conventionally and generally been used, in which anepoxy resin is cured by dicyandiamide, separation between the resin andthe filler may occur during lamination, resulting in a non-uniformlaminate. This is a characteristic phenomenon of the dicyandiamidecuring, which phenomenon is barely observed in other resin systems suchthat an epoxy resin is cured by other curing agents (a novolak resin,etc.), and this phenomenon is remarkable especially when an epoxy resinhaving a phenolic novolak skeleton or a cresol novolak skeleton is usedin a large amount. Such a non-uniform laminate has not only a problemabout appearance but also a problem in that various properties of thelaminate are degraded, especially the resistance to soldering heat ismarkedly reduced. There is an epoxy resin which does not cause theabove-mentioned separation between the resin and the filler, but, forobtaining excellent heat resistance and high reliability required forthe applications of laminate for electrical insulation and printedwiring board without using halogen elements, it is necessary to use anepoxy resin having a stiff structure, such as a phenolic novolakskeleton. For solving these problems, Japanese Unexamined PatentPublication No. 60951/1999 discloses a method in which the surface ofinorganic filler is treated with a silicone oligomer to enhance theadhesion between the resin and the filler, whereby improving thedispersion property of the resultant mixture. However, such a surfacetreatment for the filler causes the production cost to rise, and istherefore difficult to apply to general-purpose laminates and prepregs.

DISCLOSURE OF THE INVENTION

The present inventors have made intensive studies as to what actioncauses the separation between the resin and the inorganic filler, and,as a result, they have found that this problem is attributed to thecompatibility between the resin and dicyandiamide. That is,dicyandiamide is poorly compatible with most of the epoxy resins,especially epoxy resins having a phenolic novolak skeleton or a cresolnovolak skeleton, and therefore precipitates in the epoxy resin afterthe solvent volatilizes. Therefore, it is considered that the curingreaction in the prepreg becomes very non-uniform and this non-uniformitycauses the inorganic filler to be separated from the resin.

Japanese Unexamined Patent Publication No. 3283/1996, U.S. Pat. No.5,143,756, EP 0 472 830, and Japanese Unexamined Patent Publication No.140352/1993 disclose a method in which an epoxy resin and dicyandiamideare preliminarily reacted so that they are compatible with each other.However, this method is intended to make the laminate propertiesuniform, and, in the above prior art documents, an effect obtained wheninorganic filler is used in a large amount is not referred to at all.Thus, the above method does not satisfy the demand for the currentsituation in which the technique for improving the function of laminateor prepreg using filler is becoming common. Further, when a generalepoxy resin composition of dicyandiamide-curing type is merely subjectedto preliminary reaction, a problem about the storage stability of theresultant prepreg arises. For example, Japanese Unexamined PatentPublication No. 263647/1997 discloses that it is desired thatdicyandiamide is present in a dispersed state in the epoxy resin as itis incompatible with the epoxy resin, from the viewpoint of obtainingexcellent storage stability. That is, the varnish for a laminate or aprepreg using an epoxy resin and dicyandiamide needs to achieve bothexcellent compatibility in which separation of the inorganic filler fromthe resin does not occur, and excellent storage stability.

An object of the present invention is to provide a varnish for alaminate or a prepreg in which storage stability of the varnish isexcellent and separation between a resin and inorganic filler does notoccur, a flame-retardant laminate or prepreg for electrical insulationusing the varnish in which it is uniform and prevented from occurrenceof blister and has excellent heat resistance and high reliability, andfurther a printed wiring board using the laminate or the prepreg.

The present invention is solved the above-mentioned problems and aninorganic filler is introduced into an epoxy resin laminate cured bydicyandiamide with a large amount, which is uniformly dispersed therein.

That is, the present invention is a varnish for a laminate or a prepreg,which comprises: a heat treatment product which is obtained by mixingtogether (a) an epoxy resin, (b) dicyandiamide, and (c) a compoundhaving an imidazole ring so that the component (c) is present in anamount of 0.001 to 0.03% by weight, based on the weight of the component(a), and subjecting the resultant mixture to reaction for heat treatmentin an organic solvent at a temperature of 70° C. to less than 140° C. sothat all of the components are compatible with one another in theabsence of a solvent, and (d) inorganic filler; and a laminate and aprepreg obtained from the varnish as well as a printed wiring boardconstituted by using the laminate or prepreg.

BEST MODE FOR CARRYING OUT THE INVENTION

As the component (a) contained in the varnish for laminate or prepreg tobe used in the present invention, an epoxy resin having a structurerepresented by the following formula (1):

-   -   wherein each of R¹ represents a hydrogen atom or an optional        monovalent organic group, R² represents an optional divalent        organic group, and n is an integer of 2 or more,        is desired, and two or more types of the above epoxy resins can        be used in an appropriate combination.

In the formula (1), as the monovalent organic group of R¹, there may bementioned, for example, a straight or branched alkyl group having 1 to12 carbon atoms, an alkenyl group having 2 to 5 carbon atoms and thelike. R¹ is preferably a hydrogen atom or an organic group selected froma methyl group, an octyl group, a nonyl group, a decyl group, a dodecylgroup, a t-butyl group, and a vinyl group, more preferably a hydrogenatom or a methyl group. Also, as the divalent organic group of R², theremay be mentioned, for example, a substituted or unsubstituted straightor branched alkylene group having 1 to 12 carbon atoms, a substituted orunsubstituted alkenylene group having 2 to 5 carbon atoms, a substitutedor unsubstituted arylene group having 6 to 12 carbon atoms and the like.R² is preferably a divalent organic group selected from a methylenegroup, an ethylene group, a dimethyldicyclopentadienylene group, and a4,5-dimethyl-1,2-xylylene group, more preferably a methylene group.Particularly when a phenolic novolak epoxy resin produced by epoxidizingthe hydroxyl groups of a phenolic resin is used as the component (a),the effect of the present invention is remarkably exhibited.Specifically, preferred examples include a phenolic novolak epoxy resin,an ortho-cresol novolak epoxy resin, a dicyclopentadiene-modifiedphenolic novolak epoxy resin, a phenolated polybutadiene epoxy resin, apara-vinylphenolic novolak epoxy resin, and a xylene-modified phenolicnovolak epoxy resin. Of these, from the viewpoint of reducing a cost andan amount of the resin supplied, a phenolic novolak epoxy resin and anortho-cresol novolak epoxy resin are most preferred. Also, n is aninteger of 2 or more, preferably an integer of 2 to 6.

Dicyandiamide of the component (b) is a compound represented by thechemical formula of H₂NC(═NH)NHC≡N and a commercially available compoundcan be used.

Also, as the component (c), imidazole, 2- and/or 4- and/or 5-substitutedimidazoles, and N-substituted imidazoles can be used, and specifically,there can be used imidazole, 2-methylimidazole, 2-ethylimidazole,2-propylimidazole, 2-butylimidazole, 2-phenylimidazole,4-methyl-imidazole, 2,4-dimethylimidazole, 2-ethyl-4-methylimidazole,2-butyl-4-methylimidazole, 2-phenyl-4-methylimidazole, benzimidazole,1,2-dimethylimidazole, 1-methyl-2-ethylimidazole,1-benzyl-2-methylimidazole, 1-benzyl-2-phenylimidazole, a reactionproduct of imidazole and a glycidyl compound, a reaction product ofimidazole and a diglycidyl compound, and a reaction product of2-ethyl-4-methylimidazole and a diglycidyl compound. Particularly,preferred are masked imidazoles which have excellent storage stabilityat normal temperature and immediately promote a reaction upon heating,and especially preferred is a compound represented by the formula (2):

-   -   wherein each of R³, R⁴ and R⁵ represents a hydrogen atom, a        methyl group, an ethyl group or a phenyl group, and R⁶ is an        optional divalent organic group.        In the formula (2), as the divalent organic group of R⁶, there        may be mentioned, for example, a substituted or unsubstituted        straight or branched alkylene group having 1 to 12 carbon atoms,        a substituted or unsubstituted arylene group having 6 to 20        carbon atoms and the like. As the substituent, there may be        mentioned, for example, an alkyl group, an aryl group and the        like. It is preferably mentioned, for example, a 1,4-phenylene        group, a 4,4′-biphenylene group, a        3,3′,5,5′-tetramethyl-4,4′-biphenylene group, a        4,4′-diphenylenemethane group, a 2,4′-diphenylenemethane group,        a 2,2′-diphenylenemethane group, a 2,2-bis(4-phenylene)propane        group, etc., more preferably a 2,2-bis(4-phenylene)propane        group.

The component (c) promotes the curing of the epoxy resin according tothe amount thereof added, and is generally used in an amount of 0.1 to1.0% by weight, based on the weight of the epoxy resin. However, in thepresent invention, it is necessary that the amount of the component (c)be 0.001 to 0.03% by weight, based on the weight of the component (a).The reason for the use of the component (c) in such a small amount isthat a reaction for enhancing the compatibility between the epoxy resinand dicyandiamide causes the curing of the resin to be fast, and theabove-mentioned amount of the component (c) is far smaller than theappropriate amount of a curing promoter added for a general resincomposition comprising an epoxy resin and dicyandiamide. Further, thereis strong correlation between the amount of the component (c) added andthe storage stability of the varnish for a laminate or a prepreg, or theprepreg, and, from the viewpoint of obtaining excellent storagestability, the component (c) in the present invention, which secures asatisfactory curing property in a small amount, is very effective. Thatis, when the amount of the component (c) added is larger than the upperlimit of the above range, the gel time of the varnish after the reactionfor enhancing the compatibility becomes too short and the storagestability of the varnish or prepreg becomes poor. On the other hand,when the amount is smaller than the lower limit of the above range, thecuring property of the prepreg becomes poor, causing the properties ofthe resultant laminate to be degraded.

As the component (d), almost all commercially available inorganicfillers for industry can be used. In general, a laminate is produced bya method in which paper or a glass fiber base material is impregnatedwith a varnish which is obtained by dissolving a resin in an organicsolvent to prepare coated paper or coated cloth, and the prepared paperor cloth is subjected to laminating press to produce a laminate.Therefore, the component (d) needs to be uniformly dispersible in thevarnish, to be difficult to sediment or separate in the varnish, and notto greatly affect the varnish viscosity. Specifically, there can be usedone type of or a mixture of two or more types of inorganic fillersselected from aluminum hydroxide, magnesium hydroxide, silica, mica,talc, aluminum oxide, magnesium carbonate, barium carbonate, calciumcarbonate, titanium oxide, etc. Especially when using aluminum hydroxideand magnesium hydroxide which are frequently used in a large amount as aflame retardant, the present invention exhibits a remarkable effect.

The reaction between the epoxy resin of the component (a) anddicyandiamide of the component (b) is mainly a ring-opening additionreaction of the epoxy group with the amino group and imino group of thedicyandiamide, and it is known that dicyandiamide undergoes acomplicated reaction mechanism in addition to the above reaction, butcomplete elucidation of the structure of the resultant reaction producthas not yet been made. It is presumed that this reaction product hasboth the structure resulting from ring opening of an epoxy ring and thecyano group and imino group derived from dicyandiamide, and thus, thereaction product is considered to be compatible with both the epoxyresin and dicyandiamide and serve as an agent for enhancing thecompatibility between these compounds. Then, although elucidation of themechanism of the action has not yet been completed, it has been foundthat the reaction for enhancing the compatibility consequentlydramatically improves the dispersion property of the inorganic filler inthe resin and solves the problem of separation of the filler from theresin. That is, by reacting the epoxy resin of the component (a),dicyandiamide of the component (b), in the presence of the component (c)in an appropriate organic solvent, the flowability unevenness due to theseparation of inorganic filler which has been observed in the preparedlaminate is markedly improved, thus improving various properties of thelaminate. It is preferred that the reaction for enhancing thecompatibility is conducted at 70° C. to less than 140° C., andespecially at 80° C. to less than 120° C., the reaction proceeds mostefficiently. As the organic solvent to be used, an appropriate solventwhich dissolves all three components (a), (b), and (c) can be used. Assuch an organic solvent, there may be specifically mentioned methylethyl ketone, methyl isobutyl ketone, ethylene glycol, diethyleneglycol, 2-methoxyethanol, 2-(2-methoxyethyl)ethanol, 2-ethoxyethanol,2-(2-ethoxyethyl)-ethanol, N,N-dimethylformamide, N,N-dimethylacetamide,N-methyl-2-pyrrolidone, cyclohexanone, etc., and a mixture of two ormore organic solvents selected from the above solvents may be used. Thebehavior of the reaction for enhancing the compatibility between theepoxy resin and dicyandiamide varies depending on the formulation ratiothereof, but, from the viewpoint of obtaining a good balance between therate of the reaction for enhancing the compatibility and the propertiesof the cured product, it is preferred that the weight ratio of thedicyandiamide of the component (b) to the epoxy equivalent of thecomponent (a) is in terms of weight (g) of (b)/(weight (g) of (a)/epoxyequivalent)=7.35 to 11.55. When this value is larger than 11.55, a verylong time is required for the reaction for enhancing the compatibility,and, when the value is smaller than 7.35, the crosslinking density ofthe cured product is lowered, whereby causing the stiffness and heatresistance of the resultant laminate to be reduced.

In the varnish, laminate, prepreg, and printed wiring board preparedusing these of the present invention, in addition to four components(a), (b), (c), and (d), a flame retardant, a pigment, an adhesiveauxiliary, an antioxidant, and a curing promoter can be added. Further,the present invention is especially advantageously applied to ahalogen-free varnish, laminate, prepreg, and printed wiring boardprepared using these. For securing flame retardancy without using ahalogenated compound, a large amount of inorganic flame retardant isrequired to be added, and this can be easily achieved by the presentinvention. The term “halogen-free” used here means the varnish,laminate, prepreg, and printed wiring board wherein the components (a),(b), (c), and (d) and other components added do not contain chlorineand/or bromine in respective amounts of 0.1% by weight or more, based onthe total weight of the varnish, the laminate, the prepreg, or thewiring board.

Using the varnish for a laminate or a prepreg of the present invention,a prepreg and a laminate can be prepared by the same production methodas that for general laminates for electrical insulation, and a printedwiring board using the laminate or prepreg can be prepared. That is,glass cloth, glass nonwoven fabric, or organic fiber cloth isimpregnated with the varnish of the present invention, and dried byheating so that the solvent is volatilized and the curing reaction ofthe epoxy resin slightly proceeded to prepare a prepreg having a desiredcuring property and desired flowability. Also, a plurality of theprepared prepregs are stacked on one another, and covered with metalfoil on the top and bottom, and then disposed between flat heat-transferplates, and subjected to heat-press by means of a machine which can keepthe stacked prepregs at a temperature such that the curing reaction ofthe epoxy resin proceeds satisfactorily, i.e., at 150° C. or higher,preferably 170° C. or higher, thus preparing a metal-clad laminatedboard. Further, the metal foil on the metal-clad laminated board isprocessed into a circuit by a process, such as an etching process, andthen subjected to perforation, plating, and resist ink printing toprepare a printed wiring board.

EXAMPLES

In the following, the present invention will be described in more detailwith reference to Examples and Comparative examples, which should not beconstrued as limiting the scope of the present invention.

In Examples and Comparative examples, the epoxy resins, curing promotersand organic flame retardants described below were used. With respect tothe additive, inorganic filler, glass cloth, copper foil, etc., whichconstitute the laminate and prepreg, raw materials generally used in thefield of chemical industry and electronics industry were used unlessotherwise specified.

-   Epoxy resin A: Ortho-cresol novolak-type epoxy resin, N-673, trade    name (epoxy equivalent: 210), manufactured by Dainippon Ink &    Chemicals Incorporated-   Epoxy resin B: Phenolic novolak-type epoxy resin, DEN438, trade name    (epoxy equivalent: 178), manufactured by DOW PLASTICS, U.S.A.-   Curing promoter A: Reaction product of bisphenol-A-type epoxy resin    (Epikote 828, trade name, manufactured by Japan Epoxy Resins Co.,    Ltd.) and 2-ethylimidazole (2EZ, trade name, manufactured by Shikoku    Chemicals Corporation)-   Curing promoter B: 2-Ethyl-4-methylimidazole, 2E4MZ, trade name,    manufactured by Shikoku Chemicals Corporation-   Organic flame retardant A: Triphenylphosphate, TPP, trade name,    manufactured by Daihachi Chemical Industry Co., Ltd.-   Organic flame retardant B: Tricresylphosphate, TCP, trade name,    manufactured by Daihachi Chemical Industry Co., Ltd.

The properties of a laminate were evaluated as follows. The flameretardancy was evaluated in accordance with the Vertical Burning Methoddescribed in UL-Subject 94, in terms of burning time, and rating V-0indicates that the average burning time was 5 seconds or less and themaximum burning time was 10 seconds or less, rating V-1 indicates thatthe average burning time was 25 seconds or less and the maximum burningtime was 30 seconds or less, and rating HB indicates that the averageburning time and the maximum burning time were longer than those in theabove rating V-1. The resistance to soldering heat was evaluated asfollows. Test specimens (25 mm×25 mm, both surfaces copper-clad) wereindividually placed in solder baths respectively heated at 260° C. and288° C., and an occurrence of blistering was visually observed in 300seconds. The symbols shown in Table 1 indicate that: ◯: No changing; X:Blistering.

Example 1

A varnish for halogen-free laminate and prepreg was prepared inaccordance with the formulation shown below. First, an epoxy resin A,dicyandiamide, a curing promoter A, and 2-methoxyethanol were mixedtogether, and most of the dicyandiamide was dissolved at roomtemperature, and then, heated to 100° C. and reacted for 2 hours toobtain a yellow viscous uniform solution. Thereafter, the solution wascooled to 30° C. and other ingredients were added thereto to prepare avarnish. A glass cloth (weight: 105 g/m²) was impregnated with theprepared varnish, and dried at 160° C. for 4 minutes to obtain aprepreg. The eight prepregs were stacked on one another, and disposedbetween one-side roughened electrolytic copper foils each having athickness of 18 μm, followed by heat pressure molding at 185° C. at apressure of 4 MPa for 60 minutes, thus preparing a double-sidedcopper-clad laminate having a thickness of 0.8 mm.

The rate of the change between the gel time of the varnish immediatelyafter the preparation and the gel time of the varnish after storage at25° C. for 2 weeks and the properties of the laminate were evaluated,and the results are shown in Table 1. Raw materials Part(s) by weightEpoxy resin A 100 Dicyandiamide 4.5 Curing promoter A 0.006 Organicflame retardant A 15 Aluminum hydroxide 65 2-Methoxyethanol 55 Methylethyl ketone 45

Examples 2 to 5

Varnishes for laminate and prepreg were prepared, and then prepregs andlaminates were prepared in the same manner as in Example 1. Theformulations, the rates of the change in gel time, and the properties ofthe laminates are shown in Table 1.

Comparative Example 1

A varnish for laminate and prepreg was prepared, and then, a prepreg anda laminate were prepared in the same manner as in Example 1. All themixing was conducted at room temperature. A glass cloth (weight: 105g/m²) was impregnated with the prepared varnish and dried at 160° C. for4 minutes in the same manner as in Example 1 to obtain a prepreg.However, white dicyandiamide crystals were precipitated on the entiresurface of the prepreg. The eight prepregs were stacked on one anotherin the same manner as in Example 1, and disposed between one-sideroughened electrolytic copper foils each having a thickness of 18 μm,followed by heat pressure molding at 185° C. at a pressure of 4 MPa for60 minutes, thus preparing a laminate, but most of the resin flowed awayas uncured during heating and pressing, so that a laminate having adesired sheet thickness could not be prepared.

Comparative Example 2

The varnish prepared in Comparative example 1 was dried at 170° C. for20 minutes to obtain a prepreg, and then, a laminate was prepared in thesame manner as in Example 1. Precipitation of white dicyandiamidecrystals was observed on the entire surface of the prepreg, but alaminate having a desired sheet thickness could be prepared. However,when the copper foil of the prepared laminate was removed by chemicaletching, separation in a scale form between aluminum hydroxide and theresin was observed on the entire surface of the laminate. Theformulation, the rate of the change in gel time, and the properties ofthe laminate are shown in Table 1.

Comparative Example 3

A varnish for laminate and prepreg was prepared in the same manner as inExample 1. First, epoxy resin A, dicyandiamide, curing promoter A, and2-methoxyethanol were mixed together and reacted at 65° C. for 20 hours,and then, other ingredients were added thereto at room temperature. Aglass cloth (weight: 105 g/m²) was impregnated with the resultantvarnish, and dried at 160° C. for 10 minutes to obtain a prepreg.Precipitation of white dicyandiamide crystals was observed on the entiresurface of the prepreg. A laminate was prepared in the same manner as inComparative example 1. When the copper foil of the prepared laminate wasremoved by chemical etching, separation in a scale form between aluminumhydroxide and the resin was observed on the entire surface of thelaminate. The formulation, the rate of the change in gel time, and theproperties of the laminate are shown in Table 1.

Comparative Example 4

A varnish for laminate and prepreg was prepared in substantially thesame manner as in Example 1 except that the amount of curing promoter Awas changed. A glass cloth (weight: 105 g/m²) was impregnated with theprepared varnish, and dried at 160° C. for 3 minutes to obtain aprepreg, and the prepreg was subjected to heat-press in the same manneras in Example 1 to prepare a laminate. Precipitation of dicyandiamidewas not observed on the prepreg, but the flowability of the prepreg wasconsiderably lowered and satisfactory melt flow did not occur in theresin during the molding, whereby causing molding failure at the endportion of the prepreg. The formulation, the rate of the change in geltime, and the properties of the laminate are shown in Table 1. Thepreparation of a prepreg under heating conditions such that the dryingtime was 3 minutes or shorter was impossible because the solvent was notvolatilized satisfactorily. TABLE 1 (unit: part(s) by weight) Exam-Exam- Exam- Exam- Exam- Comparative Comp. Comp. Comp. ple 1 ple 2 ple 3ple 4 ple 5 example 1 ex. 2 ex. 3 ex. 4 Formulation Epoxy resin A 100100 100 100 100 100 100 100 Epoxy resin B 100 Dicyandiamide 4.5 4.5 5.34.5 4.5 4.5 4.5 4.5 4.5 Curing promoter A 0.006 0.006 0.006 0.006 0.0060.006 0.006 0.300 Curing promoter B 0.005 Organic flame 15 15 15 15 1515 15 15 retardant A Organic flame 17 retardant B Aluminum hydroxide 6565 65 65 65 65 65 65 Magnesium hydroxide 55 2-Methoxyethanol 55 55 55 5555 55 55 55 55 Methyl ethyl ketone 45 45 45 45 40 45 45 45 45 Char-Varnish gel time (sec) 480 475 423 482 491 682 682 624 199 acteristicsRetention ratio of 85 84 87 85 80 85 85 82 32 varnish gel time (%; 25°C., 2 weeks) Laminate appearance Excel- Excel- Excel- Excel- Excel- Thinspot on Scale- Scale- Excel- (after etching for lent lent lent lent lentthe entire form form lent copper foil) surface unevenness unevennessFlame retardancy 1.7 2.1 1.5 2.3 2.2 2.3 3.5 4.2 3.2 [average burningV-0 V-0 V-0 V-0 V-0 V-0 V-0 V-0 V-0 time (sec)/rating] Resistance tosoldering ◯ ◯ ◯ ◯ ◯ ◯ X ◯ ◯ heat (260° C.) Resistance to soldering ◯ ◯ ◯◯ ◯ X X X ◯ heat (288° C.)

From the results in Table 1, in all the Examples as exemplified, theseparation in a scale form between the resin and the filler, whichoccurred when a varnish was prepared by simply mixing raw materials atroom temperature, was suppressed, and a uniform laminate was obtained.Further, with respect to the resistance to soldering heat, it has beenconfirmed that blistering is suppressed and the reliability as alaminate is improved. On the other hand, in each of Comparative examples1 and 2, in which a reaction for enhancing the compatibility was notconducted, and Comparative example 3 in which a reaction for enhancingthe compatibility was conducted under conditions that are not thosespecified in the present invention, separation between the resin and thefiller occurred during the molding, so that a uniform laminate could notbe prepared. In addition, in Comparative example 4 in which the curingpromoter was added in an amount which fell outside the range specifiedin the present invention, not only could an appropriate curing propertynot be secured, but also the storage stability of the varnish wasunsatisfactory. From these results, the effectiveness and advantage ofthe present invention are apparent.

Examples 6 to 10

On the surface of the double-sided copper-clad laminate obtained in eachof Examples 1 to 5, a circuit (a test pattern) was formed by asubtractive process. Further, the surfaces of the resultant twodouble-sided copper-clad laminates having circuits were roughened toimprove the adhesion of the surfaces, and the two prepregs obtained ineach Example were together disposed between the above-roughenedlaminates, and the resultant laminate was disposed between further twoprepregs and then the both surfaces were covered with 18 μm one-sideroughened electrolytic copper foils, and subjected to laminating pressin the same manner as in Example 1, thus preparing a 6-layer printedwiring board. The prepared printed wiring board was subjected toexterior circuit processing, through hole formation, resist inkprinting, and parts mounting by a general method. It has been confirmedthat the printed wiring board can be processed by an ordinary productionprocess for a general printed wiring board without a problem, and thatthe resultant printed wiring board meets the quality and reliabilityrequired for the glass epoxy laminate provided in JIS C 6484 and 6486.

UTILIZABILITY IN INDUSTRY

According to the present invention, a varnish for a laminate or aprepreg in which an epoxy resin and dicyandiamide have goodcompatibility, causes no separation between the resin and the filler andhave good storage stability simultaneously, a flame-retardant laminateor prepreg for electrical insulation using the varnish in which it isuniform and prevented from occurrence of blister and has excellent heatresistance and high reliability, and a printed wiring board using thelaminate or the prepreg can be provided.

1. A varnish for laminate or prepreg, comprising: a heat treatmentproduct which is obtained by mixing together (a) an epoxy resin, (b)dicyandiamide, and (c) a compound having an imidazole ring so that thecomponent (c) is present in an amount of 0.001 to 0.03% by weight, basedon the weight of the component (a), and subjecting the resultant mixtureto reaction for heat treatment in an organic solvent at a temperature of70° C. to less than 140° C. so that all of the components are compatiblewith one another in the absence of a solvent; and (d) inorganic filler.2. The varnish for laminate or prepreg according to claim 1, wherein thecomponent (a) has a structure represented by the following formula (1):

wherein each of R¹ represents a hydrogen atom or an optional monovalentorganic group, R² represents an optional divalent organic group, and nis an integer of 2 or more.
 3. The varnish for laminate or prepregaccording to claim 2, wherein the component (a) is one or two kinds ofepoxy resin selected from the group consisting of a phenolicnovolak-type epoxy resin and an ortho-cresol novolak-type epoxy resin.4. The varnish for laminate or prepreg according to claim 1, wherein thecomponent (d) is at least one inorganic filler selected from the groupconsisting of aluminum hydroxide, magnesium hydroxide, silica, mica,talc, aluminum oxide, magnesium carbonate, barium carbonate, calciumcarbonate, and titanium oxide.
 5. The varnish for laminate or prepregaccording to claim 4, wherein the reaction for the components (a), (b),and (c) is conducted in at least one organic solvent selected from thegroup consisting of methyl ethyl ketone, methyl isobutyl ketone,ethylene glycol, diethylene glycol, 2-methoxyethanol,2-(2-methoxyethyl)ethanol, 2-ethoxyethanol, 2-(2-ethoxyethyl)ethanol,N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone,and cyclohexanone.
 6. The varnish for laminate or prepreg according toclaim 5, wherein the ratio of the weight of the dicyandiamide of thecomponent (b) to the epoxy equivalent of the component (a) is 7.35 to11.55, in terms of [weight (g) of (b)]/{[weight (g) of (a)]/epoxyequivalent}.
 7. A laminate or prepreg prepared using the varnishaccording to claim
 6. 8. A printed wiring board prepared using at leastone of the laminate and the prepreg according to claim
 7. 9. The varnishaccording to claim 6, wherein the four components (a), (b), (c), and (d)and the other components added do not contain chlorine and bromine inrespective amounts of 0.1% by weight or more, based on the weight of thevarnish.
 10. The laminate or prepreg according to claim 7, wherein thefour components (a), (b), (c), and (d) and other components added do notcontain chlorine and bromine in respective amounts of 0.1% by weight ormore, based on the weight of the laminate or prepreg.
 11. The printedwiring board according to claim 8, wherein the four components (a), (b),(c), and (d) and other components added do not contain chlorine andbromine in respective amounts of 0.1% by weight or more, based on theweight of the printed wiring board.
 12. The varnish for laminate orprepreg according to claim 1, wherein the component (a) is one or twokinds of epoxy resin selected from the group consisting of a phenolicnovolak-type epoxy resin and an ortho-cresol novolak-type epoxy resin.13. The varnish for laminate or prepreg according to claim 1, whereinthe reaction for the components (a), (b), and (c) is conducted in atleast one organic solvent selected from the group consisting of methylethyl ketone, methyl isobutyl ketone, ethylene glycol, diethyleneglycol, 2-methoxyethanol, 2-(2-methoxyethyl)ethanol, 2-ethoxyethanol,2-(2-ethoxyethyl)ethanol, N,N-dimethylformamide, N,N-dimethylacetamide,N-methyl-2-pyrrolidone, and cyclohexanone.
 14. The varnish for laminateor prepreg according to claim 1, wherein the ratio of the weight of thedicyandiamide of the component (b) to the epoxy equivalent of thecomponent (a) is 7.35 to 11.55, in terms of [weight (g) of (b)]/{[weight(g) of (a)]/epoxy equivalent}.
 15. A laminate or prepreg prepared usingthe varnish according to claim
 1. 16. A printed wiring board preparedusing at least one of the laminate and the prepreg according to claim15.
 17. The varnish according to claim 1, wherein the four components(a), (b), (c), and (d) and the other components added do not containchlorine and bromine in respective amounts of 0.1% by weight or more,based on the weight of the varnish.
 18. The laminate or prepregaccording to claim 15, wherein the four components (a), (b), (c), and(d) and other components added do not contain chlorine and bromine inrespective amounts of 0.1% by weight or more, based on the weight of thelaminate or prepreg.
 19. The printed wiring board according to claim 16,wherein the four components (a), (b), (c), and (d) and other componentsadded do not contain chlorine and bromine in respective amounts of 0.1%by weight or more, based on the weight of the printed wiring board.